Subscriber line interface circuits (SLIC), customarily found in a central office exchange of a telecommunications network, integrate digital switching networks of the central office exchange to the analog subscriber lines. The analog subscriber lines connect to subscriber stations or instruments found at subscriber locations remote from the central office exchange. SLICs function to supply power to a subscriber station and to transmit and receive voice signals between the digital switching network and the subscriber station.
As an interface between the SLIC and the digital switching system, a codifier/decodifier (CODEC) filter translates analog voice signals into encoded digital signals. The CODEC filter converts analog voice signals received from a subscriber line into encoded digital signals. Similarly, The CODEC filter converts encoded digital signals from the digital switching system into analog voice signals for transmission on the subscriber line.
Impedance mismatch at the telephone central office between the subscriber lines/trunks line and the terminating impedance within a digital switching network is well known. The two-to-four wire conversion between the subscriber line and terminating impedance causes this impedance mismatch which results in poor return loss characteristics and reflections or echoes. To correct these deficiencies requires the line impedance and the terminating impedance to be equal in both phase and magnitude over the bandwidth of the telephone channel.
The CODEC filter must effectively and efficiently perform analog-to-digital and digital-to-analog conversion, power level adjustment, and impedance matching for a subscriber line interface circuit.
Passive fixed terminating impedances to date represent a compromise, except at a specific frequency, due to the fact that such compromise impedance is either a series or parallel combination of a resistor and capacitor. Such a simple impedance results in a poor match for the line impedance. Typically, a hybrid which performs two-to-four wire conversion depends upon a relatively close match between the line and the terminating impedances and for optimum performance. Compensating for this mismatch has been attempted in the prior art with balance or “building-out” networks which are a part of the hybrid, and which, for a given line, represents a custom tailoring of the circuit.
The latest approach to provide impedance matching is derived by Advanced Micro Device (AMD) as is shown in FIG. 1. The impedance matching network of subscriber line device block diagram 100 includes an Analog Impedance Scaling Network (AISN) 126, having a programmable analog gain of −0.9375 to +0.9375, and programmable digital filter (Z) 110. Filter 110 is a programmable digital filter providing an additional path and programming flexibility over the AISN 126 to modify the transfer function from the path corresponding to nodes Vin and Vout. This digital line circuit eliminates the previously associated discrete analog components. It also provides automatic digital synthesis of line terminating impedance for any transmission line characteristic, eliminating the previously used analog components. These features make it possible to implement complete electronic hybrid and impedance matching functions as part of an overall CODEC function, enabling these functions to be implemented on a single LSI chip without extensive analog external balancing or adjusting circuitry. This results in a low cost, low power, high density and a highly reliable telecommunications line-circuit. Z-filter 110 has the following transfer function:Hz(z)=z0+z1z−1+z2z−2+z3z−3+z4z−4+(z5z6z7z−1)
Software calculates the Z-filter coefficients of digital filter 110 to maximize the return loss. Difficulty, however, lies in the complexity and time associated with the derivation of each Z-filter coefficient.
Thus, there exists a need for CODEC filter that performs impedance matching without the complex calculation of z-filter coefficients.